1. Field of the Invention
The present invention relates to the repair of gas turbine engines and, more particularly, to methods and apparatus for removing damaged or worn vanes from a compressor stator during engine overhaul.
2. Description of the Prior Art
Generally, the compressor stator of a gas turbine engine includes concentric inner and outer cylindrical shroud members having a plurality of airfoil shaped vanes fixtured therebetween such that the longitudinal axis of each vane is radially aligned relative to the shroud members. In certain engines, the vanes are fixtured by brazing or otherwise metallurgically attaching the opposite ends of each vane in suitably shaped slots through the inner and outer shroud members. In the past, expensive noble metal alloys, such as 82% Au-18% Ni and 54% Ag-25% Pd-21% Cu, have been primarily utilized as braze alloys. Nickel base braze alloys, although much less expensive, have found only limited use.
Vane replacement is a common overhaul repair for gas turbine engine compressor stators. Damaged vanes are removed either singly or as a set and new vanes rebrazed in their place so that the compressor stator can be returned to service. Methods now available for removal of damaged vanes include oxyacetylene torching, electrical discharge machining, chemical dissolution and mechanical grinding. In oxyacetylene torching, the damaged vane is heated until the braze alloy softens sufficiently to allow the vane to be physically extracted with pliers. This removal technique is disadvantageous in that, due to the high localized temperature required to soften the braze alloy, the slots through the shroud members become oxidized and deformed and must be remachined. Also, due to the heat, the shroud alloy exhibits evidence of over-tempering with local variations in ductility and hardness. As a result, compressor stators repaired by this technique normally experience a reduction in structural strength. The electrical discharge machining technique utilizes an airfoil shaped electrode to remove the attachment areas. This technique is expensive in that 20 to 30 minutes are required to remove each vane and it is relatively imprecise in that machining often does not follow the exact outline of the shroud slot, resulting in oversized slots which are very difficult to rebraze, which exhibit reduced joint strength and which can cause changes in vane angle. Further, the machining process creates a recast alloy layer in the shroud slot which, if not properly treated, will lead to shroud cracking. In chemical removal techniques, localized areas of the shroud are submerged in a tank of suitable acid or the like to dissolve the braze alloy from the slot and vane. Of course, this process is disadvantageous since other areas of the stator must be masked to prevent contact with the acid. For certain braze alloys, very long times of submersion are required to effect removal of the braze material. Finally, mechanical grinding involves manually grinding the vane flush with both the inner and outer shrouds and then grinding out the vane remnants from the shroud slots. This vane removal process is not only time consuming, but also includes a high probability for damaging the shroud slots and surrounding shroud areas. A primary reason why nickel base braze alloys have found only limited use for vane attachment in compressor stators is the unsatisfactoriness of the above-described techniques in removing such a brazing material during engine overhaul operations.